The striker fired in his 24th goal of the season just four minutes after the interval to cancel out Alex Revell's first-half opener for Steve Evans' side.
Garner's effort was of the highest quality, the 26-year-old taking two touches without the ball bouncing before arrowing a wonderful effort into the top corner from 20 yards.
That came after Revell had given Rotherham, who finished just one point ahead of the hosts in the regular season, the lead with a cool finish following a powerful run.
It is the third time that the two sides have drawn this season, with February's meeting at Deepdale ending 3-3 and the reverse fixture finishing goalless.
Preston manager Simon Grayson is eyeing his fourth promotion from the third tier in seven years, having achieved the feat with Blackpool, Leeds United and Huddersfield Town.
Britt Assombalonga's 33rd goal of the season in all competitions was eclipsed by Moses Odubajo's second-half equaliser as Leyton Orient claimed a 1-1 draw at Peterborough United in the other semi.
Former Southend United striker Assombalonga put Darren Ferguson's side on course for a second Wembley visit of the season, with the club having won the Football League Trophy final last month.
Assombalonga was a scorer in that game, too, and his close-range header - which had a hint of offside about it - handed United the initiative at London Road.
But Orient scored a deserved equaliser as a sweeping move was capped by Odubajo's tap-in after Robert Olejnik had parried David Mooney's effort.
Orient host Peterborough on Tuesday, while Rotherham entertain Preston on Thursday.
